At Fuse Fitness in Berkeley, California, founders Kristin Rios and Pascha Brown, both certified personal trainers, said they ease beginners into exercise.

“With beginners, we have them start with just two workouts a week, so they have plenty of time to rest and recover in between,” Rios and Brown told Healthline. “We encourage walking and yoga between sessions, and plenty of good sleep and water to help with soreness.”

The workouts at Fuse Fitness are hour-long sessions that pack in all the key elements of fitness — mobility and flexibility, strength training, conditioning, agility, balance, and core training.

This saves you time, which is nice if you’re short on that.

As people become more comfortable exercising, Rios and Brown encourage them to increase to three to four hour-long sessions a week. These sessions can be either one-on-one workouts or group classes.

Likewise, Walls said that “over the span of one to two years, the intensity should be increased as the person becomes more comfortable with doing the movements correctly and their fitness level improves.”

Working out at the gym, though, is not the only way to stay active.

“We encourage our clients to participate in activity every day when they’re not training with us,” said Rios and Brown, “whether it’s hiking, bike riding, gardening, rock climbing, or yoga.”

These non-gym activities can easily get you up to the 150 minutes a week of aerobic activity.

Just don’t neglect whole-body strength training for more aerobic activity. Stronger muscles burn calories, strengthen your bones and make it easier to stay active throughout your life.

It’s OK to do aerobic activity and resistance exercise on the same day. Walls recommends that if you’re doing a traditional resistance training session, do it before aerobic exercise. Circuit training and some fitness classes, though, mix things up more, with good results.

To avoid fitness burnout, take off one day a week from structured exercise. Go for a hike or bike ride, hit the beach, or walk around your city.

How hard should I exercise?

One way to tell if you’re working hard enough is to just “feel” it.

“All activities should be done at a level which challenges the individual. Cardio should make us huff and puff. Strength training should make us grunt a little. Stretches should make us wince. Balance training should make us wobble,” said Fitz Koehler, a fitness and sport performance expert with a master’s degree in exercise and sport sciences.

Koehler added that if you can hardly talk, you’re probably working too hard.

Activities like brisk walks, baseball, and some types of yoga are generally moderate intensity.

Martial arts, bicycling at a good pace, and basketball are usually vigorous.

Heart rate and activity monitors can also tell you how hard you are working. Check your device’s instructions to know how to figure that out.

Walls recommends that people try doing aerobic exercise with and without one of these devices.

“As people become more used to knowing how hard they have to work to get their heart rate up to ideal ranges, the monitor can be used but is not as necessary,” said Walls.

Rios and Brown said that a heart rate monitor is also “a great way to stay motivated and inspired to work hard. It allows you to know when you can push harder (on those lazy days).”

Once you have a solid base of fitness, you can try increasing the intensity even more.

“In HIIT, you work as hard as you can for a short amount of time, anywhere from 20 to 40 seconds,” said Rios and Brown. “Then you rest for a short interval to allow your heart rate to come down a bit and prepare for the next interval of work.”

Rinse and repeat for 10 to 30 minutes.

Many people are attracted to HIIT because you get the same benefits in less time as you would with a longer, less-intense workout.

But don’t overdo it — give yourself at least one day in between your HIIT workouts to recover.

“People often forget this part,” said Rios and Brown. “The body actually gets stronger during your recovery time. If you continually work at a high level of intensity without taking any rest days, you will leave yourself very vulnerable to injuries and/or overtraining.”

How do I start exercising safely?

If you’re just starting out, give your body time to adapt to the new movements and activities.

“Training should always progress gradually,” Koehler told Healthline.

If you do too much, too fast, you risk burning out or even injuring yourself.

A good way to look at the physical activity guidelines is as a long-term goal.

You may start to see changes in your energy levels after a few weeks of regular exercise, but the big shifts in your fitness can take a year or longer.

So start with what you can realistically handle — both physically and time-wise.

The longer you stick with exercise, the more likely it will become a habit.

One 2009 study in the European Journal of Social Psychology found that it takes on average 66 days to build a new habit.

So choose a plan that you can achieve each and every day.

That might mean walking moderately for 20 minutes three days a week.

Once you are comfortable with that, try doing one day of resistance training each week. Then two. Then three.